The close association between clubbing of the fingers and some forms of heart and lung disease have been reviewed fully by Mendlowitz (1942). The frequent association of clubbing of the extremities with cyanotic heart disease led to the suggestion that hypoxia was an important causal factor (Mendlowitz, 1942), but hypoxia alone does not account for the presence of finger clubbing in many types of lung disease. Cudkowicz and Wraith (1957a) could find no single clinical common factor in 27 patients with finger clubbing and various pulmonary disorders. From the anatomical standpoint, Cudkowicz and Armstrong (1953) suggested that the essential feature in thelungs of patients with clubbing was the presence of precapillary communications between the bronchial and pulmonary arteries. Later they supported this suggestion with right heart catheterization studies in 28 patients with finger clubbing and various lung disorders; they found high oxygen saturations in the pulmonary artery branch to the diseased lobe and concluded that there must be a left to right shunt at pulmonary artery level. More recently Cudkowicz, Calabresi, Nims, and Gray (1959) have shown in four patients with clubbing that the output of the left ventricle exceeded that of the right, and they regarded this as further evidence in favour of a broncho-pulmonary shuntin the lung. But there is an alternative explanation for the greater output of the left ventricle which does not invoke a pre-capillary bronchopulmonary shunt. In the present work the finer detail of the systemic vessels to the lung has been studied using a microradiographic technique which has been specially developed to demonstrate bronchial vessels. It will be shown that although systemic-pulmonary anastomoses are commonly found in the lungs from patients with finger clubbing, expansion of the systemic vascular bed within the lungs is a more constant feature. Although it has been widely recognized that the bronchial arteries enlarge in many lung diseases, it has been insufficiently appreciated that there is a wide variation in the patterns of this vascular increase, and any of these are characteristic of the underlying disease (Turner-Warwick, 1961). Preliminary evidence is also presented which suggests that the peripheral vascular changes, identified in-the fingers as clubbing, may be part of a widespread vascular change affecting many tissues.